As backlight devices for liquid crystal displays, edge-light type backlight devices and direct-type backlight devices have been conventionally used. The edge-light type backlight device is a system wherein a light source is disposed on a light incident edge at a side of an optical waveguide and a light entering from the light incident edge is reflected in the optical waveguide and emitted from a principal surface (light emitting plane) of the optical waveguide. Meanwhile, the direct-type backlight device is a system wherein a light source is disposed on a back surface of the light emitting plane on a light diffusing plate. A commonly used example of such a direct-type backlight device is the one having a plurality of linear light sources arranged in approximately parallel, a reflecting plate which reflects the light from these linear light sources, and a light diffusing member which receives, on a light incident plane thereof, direct light from the linear light sources and reflected light from the reflecting plate, wherein the light is then diffused and then goes out of the member from a light outgoing plane thereof which is an opposite surface of the light incident plane.
In such a direct-type backlight device, the linear light sources are disposed directly under the light diffusing member. Thus, there is a tendency of high luminance at a position directly above the linear light source, and the luminance tends to be reduced as distance from this directly-above position becomes longer. Such a tendency causes cyclic luminance unevenness on the light emitting plane. Thus, for example, Patent Document 1 discloses a direct-type backlight device having a predetermined prism row formed on a surface of the light diffusing plate, thereby having a high luminance and suppressed luminance unevenness. In Example in Patent Document 1, there is described an example wherein a reflecting plate having a flat reflecting plane was used and the distance between centers of the linear light sources was 25 mm.
Patent Document 1: International Publication No. WO2006/022270 Pamphlet